Barad Eithel, at the source of the Sirion in the Ered Wethrin, was a chief fortress of Fingolfin and his son Fingon,[1] which guarded the pass into their realm of Mithrim. Further south, in the Pass of Sirion, lay Tol Sirion (see below). After the Coming of Men, the Edain of the House of Haleth made their home in the Forest of Brethil,[4] and the Sindar ruled by Thingol lay secure within the Girdle of Melian in their realm of Doriath.[5] At the outflow of Sirion into Belegaer, after the destruction of Eglarest and Brithombar, the Havens of Sirion were built by Círdan and his people.[6]

The most important island in Sirion was Tol Sirion.[2] The original Minas Tirith was built here by Finrod Felagund in the strategic location controlling the Pass of Sirion where it entered Beleriand between the Ered Wethrin and the Echoriath.[1] Shortly after Dagor Bragollach it was captured by Sauron and the island became known as Tol-in-Gaurhoth.[7]

Sirion is a Sindarin/Noldorin name which contains the element sîr.[8] Depending on the meaning of the second element, the name could either mean "great river" or "land of waters" (-ion as augmentative suffix or archaic plural genitive suffix).[9]